How to run outstanding tasks immediately after ExecutorService.shutdown()?

Question

I've got a ScheduledExecutorService with tasks scheduled to execute in an hour. How do I get the list of outstanding tasks so I can force them to run immediately?

I believe shutdown() will wait an hour and it looks as if shutdownNow() returns a list of Runnables that cannot be run() because the Runnable implementation checks the Executor state and when it notices that it has shut down the Runnable refuses to run. See ScheduledThreadPoolExecutor.ScheduledFutureTask.run() for the actual implementation.

Any ideas?

Answer 1

I've taken Mark Peters' answer, implementing all abstract methods, added thread-safety and tried respecting the underlying ScheduledThreadPoolExecutor configuration whenever possible.

/**
 * Overrides shutdown() to run outstanding tasks immediately.
 * 
 * @author Gili Tzabari
 */
public class RunOnShutdownScheduledExecutorService extends AbstractExecutorService
    implements ScheduledExecutorService
{
    private final ScheduledExecutorService delegate;
    private final ScheduledThreadPoolExecutor scheduledThreadPoolExecutor;
    private final ExecutorService immediateService;
    private final ConcurrentMap<Future<?>, Callable<?>> tasks = Maps.newConcurrentMap();

    /**
     * Creates a new RunOnShutdownScheduledExecutorService.
     * 
     * @param delegate the executor to delegate to
     */
    public RunOnShutdownScheduledExecutorService(ScheduledExecutorService delegate)
    {
        Preconditions.checkNotNull(delegate, "delegate may not be null");

        this.delegate = delegate;
        if (delegate instanceof ScheduledThreadPoolExecutor)
        {
            this.scheduledThreadPoolExecutor = (ScheduledThreadPoolExecutor) delegate;
            this.immediateService = Executors.newFixedThreadPool(scheduledThreadPoolExecutor.
                getCorePoolSize(), scheduledThreadPoolExecutor.getThreadFactory());
        }
        else
        {
            scheduledThreadPoolExecutor = null;
            this.immediateService = Executors.newSingleThreadExecutor(new ThreadFactoryBuilder().
                setNameFormat(RunOnShutdownScheduledExecutorService.class.getName() + "-%d").build());
        }
    }

    @Override
    public boolean isShutdown()
    {
        return delegate.isShutdown();
    }

    @Override
    public boolean isTerminated()
    {
        return delegate.isTerminated();
    }

    @Override
    public boolean awaitTermination(long timeout, TimeUnit unit) throws InterruptedException
    {
        long before = System.nanoTime();
        if (!delegate.awaitTermination(timeout, unit))
            return false;
        long after = System.nanoTime();
        long timeLeft = timeout - unit.convert(after - before, TimeUnit.NANOSECONDS);
        return immediateService.awaitTermination(timeLeft, unit);
    }

    @Override
    public void execute(Runnable command)
    {
        delegate.execute(command);
    }

    @Override
    public ScheduledFuture<?> schedule(final Runnable command, long delay, TimeUnit unit)
    {
        CleaningRunnable decorated = new CleaningRunnable(command);
        ScheduledFuture<?> future = delegate.schedule(decorated, delay, unit);
        decorated.setFuture(future);
        tasks.put(future, Executors.callable(command));
        return new CleaningScheduledFuture<>(future);
    }

    @Override
    public <V> ScheduledFuture<V> schedule(Callable<V> callable, long delay, TimeUnit unit)
    {
        CallableWithFuture<V> decorated = new CallableWithFuture<>(callable);
        ScheduledFuture<V> future = delegate.schedule(decorated, delay, unit);
        decorated.setFuture(future);
        tasks.put(future, callable);
        return new CleaningScheduledFuture<>(future);
    }

    @Override
    public ScheduledFuture<?> scheduleAtFixedRate(Runnable command, long initialDelay, long period,
        TimeUnit unit)
    {
        CleaningRunnable decorated = new CleaningRunnable(command);
        ScheduledFuture<?> future = delegate.scheduleAtFixedRate(decorated, initialDelay, period, unit);
        decorated.setFuture(future);
        tasks.put(future, Executors.callable(command));
        return new CleaningScheduledFuture<>(future);
    }

    @Override
    public ScheduledFuture<?> scheduleWithFixedDelay(Runnable command, long initialDelay, long delay,
        TimeUnit unit)
    {
        CleaningRunnable decorated = new CleaningRunnable(command);
        ScheduledFuture<?> future =
            delegate.scheduleWithFixedDelay(decorated, initialDelay, delay, unit);
        decorated.setFuture(future);
        tasks.put(future, Executors.callable(command));
        return new CleaningScheduledFuture<>(future);
    }

    @Override
    public synchronized void shutdown()
    {
        if (delegate.isShutdown())
            return;
        if (scheduledThreadPoolExecutor != null)
        {
            // WORKAROUND: http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=7069418
            //
            // Cancel waiting scheduled tasks, otherwise executor won't shut down
            scheduledThreadPoolExecutor.setExecuteExistingDelayedTasksAfterShutdownPolicy(false);
        }
        delegate.shutdown();
        // Users will not be able to cancel() Futures past this point so we're guaranteed that
        // "tasks" will not be modified.

        final List<Callable<?>> outstandingTasks = Lists.newArrayList();
        for (Map.Entry<Future<?>, Callable<?>> entry: tasks.entrySet())
        {
            Future<?> future = entry.getKey();
            Callable<?> task = entry.getValue();

            if (future.isDone() && future.isCancelled())
            {
                // Task called by the underlying executor, not the user. See CleaningScheduledFuture.
                outstandingTasks.add(task);
            }
        }
        tasks.clear();
        if (outstandingTasks.isEmpty())
        {
            immediateService.shutdown();
            return;
        }

        immediateService.submit(new Callable<Void>()
        {
            @Override
            public Void call() throws Exception
            {
                delegate.awaitTermination(Long.MAX_VALUE, TimeUnit.DAYS);

                // Execute outstanding tasks only after the delegate executor finishes shutting down
                for (Callable<?> task: outstandingTasks)
                    immediateService.submit(task);
                immediateService.shutdown();
                return null;
            }
        });
    }

    @Override
    public List<Runnable> shutdownNow()
    {
        return delegate.shutdownNow();
    }

    /**
     * A Runnable that removes its future when running.
     */
    private class CleaningRunnable implements Runnable
    {
        private final Runnable delegate;
        private Future<?> future;

        /**
         * Creates a new RunnableWithFuture.
         * 
         * @param delegate the Runnable to delegate to
         * @throws NullPointerException if delegate is null
         */
        public CleaningRunnable(Runnable delegate)
        {
            Preconditions.checkNotNull(delegate, "delegate may not be null");

            this.delegate = delegate;
        }

        /**
         * Associates a Future with the runnable.
         * 
         * @param future a future
         */
        public void setFuture(Future<?> future)
        {
            this.future = future;
        }

        @Override
        public void run()
        {
            tasks.remove(future);
            delegate.run();
        }
    }

    /**
     * A Callable that removes its future when running.
     */
    private class CallableWithFuture<V> implements Callable<V>
    {
        private final Callable<V> delegate;
        private Future<V> future;

        /**
         * Creates a new CallableWithFuture.
         * 
         * @param delegate the Callable to delegate to
         * @throws NullPointerException if delegate is null
         */
        public CallableWithFuture(Callable<V> delegate)
        {
            Preconditions.checkNotNull(delegate, "delegate may not be null");

            this.delegate = delegate;
        }

        /**
         * Associates a Future with the runnable.
         * 
         * @param future a future
         */
        public void setFuture(Future<V> future)
        {
            this.future = future;
        }

        @Override
        public V call() throws Exception
        {
            tasks.remove(future);
            return delegate.call();
        }
    }

    /**
     * A ScheduledFuture that removes its future when canceling.
     * 
     * This allows us to differentiate between tasks canceled by the user and the underlying
     * executor. Tasks canceled by the user are removed from "tasks".
     * 
     * @param <V> The result type returned by this Future
     */
    private class CleaningScheduledFuture<V> implements ScheduledFuture<V>
    {
        private final ScheduledFuture<V> delegate;

        /**
         * Creates a new MyScheduledFuture.
         * 
         * @param delegate the future to delegate to
         * @throws NullPointerException if delegate is null
         */
        public CleaningScheduledFuture(ScheduledFuture<V> delegate)
        {
            Preconditions.checkNotNull(delegate, "delegate may not be null");

            this.delegate = delegate;
        }

        @Override
        public long getDelay(TimeUnit unit)
        {
            return delegate.getDelay(unit);
        }

        @Override
        public int compareTo(Delayed o)
        {
            return delegate.compareTo(o);
        }

        @Override
        public boolean cancel(boolean mayInterruptIfRunning)
        {
            boolean result = delegate.cancel(mayInterruptIfRunning);

            if (result)
            {
                // Tasks canceled by users are removed from "tasks"
                tasks.remove(delegate);
            }
            return result;
        }

        @Override
        public boolean isCancelled()
        {
            return delegate.isCancelled();
        }

        @Override
        public boolean isDone()
        {
            return delegate.isDone();
        }

        @Override
        public V get() throws InterruptedException, ExecutionException
        {
            return delegate.get();
        }

        @Override
        public V get(long timeout, TimeUnit unit) throws InterruptedException, ExecutionException,
            TimeoutException
        {
            return delegate.get(timeout, unit);
        }
    }
}